1. Field of the Invention
This invention relates to arc furnace electrodes having a top portion and a separate replaceable bottom portion of a consumable material, the portions being interconnected by means of a screw nipple or the like. The top portion is provided with a liquid cooling device having feed and return ducts. The top portion is provided with a protective covering.
2. Description of the Prior Art
Arc electrodes are known and are shown for example in Belgian Pat. No. 867 876. In such electrodes a metal shank, containing a cooling system, is covered by an externally disposed, substantially temperature resistant compound. This is evidently a continuous covering, adhesion to the metal shank is improved by hooks which are included in the metal shank.
British Pat. No. 1 223 162 shows electrodes in which the entire metallic shank is covered with a protective ceramic coating. The ceramic coating is applied as thinly as possible and penetrates into the metal shank to insulate a substantial portion of the tube extending therein. These tubes also provide cooling water ducting and an electrical conduit to the consumable electrode portion.
European Patent Application 79 302 809.3 describes an electrode in which a metallic contact of a metal shank, disposed laterally on the outside, is supported so as to be insulated with respect to an internally disposed cooling system. Bottom portions of the cooling shank is provided with a ceramic coating, secured by means of hooks, which extending approximately to the height of the screw nipple connection.
Electrodes for arc furnaces are exposed to severe stresses at least in part due to elevated or high operating temperatures, for example, in the production of electro-steel, where such electrodes are most frequently employed. Losses due to side oxidation occur as a result of the operating conditions within the furnace and by virtue of the arc which extends into the melt from the bottom electrode tip only in an ideal case. Lateral striking of the arc can also occur above the consumable portions of the electrode and can lead to short circuits. The electrodes are subjected to a temperature differential between zones of coolant feed and return as well as between the consumable part contrasted to the electrical current supplying and cooling portion. Substantial additional mechanical stresses occur when the electrodes enter the charge and as a result of the addition of scrap into the melt.
German Auslegeschrift No. 27 30 884 also describes electrodes for arc furnaces having a sheath mounted on an electrode support and containing liquid cooled metal components and including a core axially slidable within the sheath. The sheath mounted on the electrode support arm is defined with respect to the core, which slides therethrough, by an insulating stratum between the sheath and core. A device for generating a magnetic field is provided in the bottom part of the sheath. Such electrodes have different advantages, due on the one hand to a "chimney effect" which causes side burning, although somewhat reduced by contrast to conventional graphite electrodes. The temperature of the melt also acts over a longer region within such a sliding electrode so that the oxidation sensitivity of the consumable graphite portion is increased. Finally, the operation of such electrode requires a relatively large supply of energy.
German Auslegeschrift No. 28 45 367 also discloses liquid cooling retaining means for an electrode tip, wherein a heat shield comprising a metal tube is provided, electrically insulated with respect to the current carrying cooling system and cooled via a refractory compound which is rammed or case between the cooling system and the metal tube. The metal tube thus described is already known from German patent specification No. 807 312. Ramming or casting refractory compounds into the space between the metal tube and the cooling system in accordance with German Auslegeschrift No. 28 45 367 and results in a non-detachable connection having substantial disadvantages in the event of thermal shock or in connection with maintenance or repair work.